The disclosed invention relates to semiconductor device processing, and more particularly relates to a process for creating flip-chip conductive-polymer bumps using conventional photolithography and polishing techniques.
Conductive adhesive material technology has been extensively explored in the last decade as an alternative to replace conventional metallic solders in flip-chip packaging. Conductive adhesives offer several advantages as compared to metallic solders, for example, the fabrication techniques for such interconnects are simple, fast, and inexpensive. These materials are lead-free, have a superior environmental compatibility, and can be used with a wide range of surfaces including non-solderable surfaces. The lower temperatures required for processing result in lower thermal stress, and the lower thermo-mechanical fatigue results in a good temperature cycling performance. Further, thermoplastic conductive polymer interconnects can be reworked and reformed. Such materials also have the ability to absorb, and thus minimize, the stresses caused due to coefficient of thermal expansion (CTE) mismatches between dissimilar flip-chip mating surfaces.
Conventional commercial conductive adhesive manufacturing technology is based on screen-printing processes. While this approach can meet many demands of some applications, the next generation of semiconductor devices and systems will require processes that can enable cost-effectiveness and higher-performance levels. This requires development of a process that can enable definition of interconnects with higher resolution as compared to screen-printing, to enable finer pitch capability and greater control over the bump profile (e.g. bump height, size, and surface uniformity). Surprisingly little work has been done to address this problem.
As an example of conventional conductive polymer techniques, Kwang W. Oh, and Chon H. Ahn have described “A New Flip-Chip Bonding Technique Using Micromachined Conductive Polymer Bumps,” IEEE Transactions on Advanced Packaging, Vol.22, No 4, November 1999. In this reference, the authors use a positive resist for photolithography and demonstrate bump formation, however their process does not describe a method which easily and repeatably dispenses and scrapes the polymer material. Also, hard baking the molds of positive resist at 150 C or 230 C results in their reduced solubility in acetone, making them difficult to strip.
C. K. Wong, O. C. Cheung, B. Xu, and M. M. Yuen have described a similar process, “Using PDMS Microtransfer molding for polymer flip-chip,” Proceedings of the Electronic Components and Technology Conference (ECTC), IEEE, 652–657, (2003). In this approach, polydimethysiloxane micro-transfer templates were used to define the bumps.
In both of these approaches, however, the key to attaining good selectivity between the conductive adhesive and the photoresist templates was selective curing of both materials after squeegee-scraping excess adhesive from over the molds. These approaches used a high-viscosity conductive adhesive for defining the bumps. Consequently, mass-fabrication of conductive adhesive interconnects using both these approaches requires the development of specialized, and possibly trouble-prone head equipment for dispensing of the thick conductive adhesive, and for removing excess material from over the photoresist templates.
Additionally, using photoresist templates and squeegee for bump definition can result in a meniscus effect (bowing due to squeegee scraping over soft/flexible templates), which can cause a reduction in the effective surface area between the mating bump and pad, thereby resulting in a higher contact resistance and possibly reduced joint reliability.
Further, conventional micro/optoelectronic packaging industry polymer flip-chip wafer-bumping processes use screen-printing or stencil-printing techniques that offer relatively crude alignment resolution of approximately 10 μm. When high-viscosity polymers are used, silk-screens are needed, along with a relatively sophisticated head system for scraping and dispensing the polymer material by use of a polishing technique is required.
In general, lead-free conductive polymers are preferred materials for flip-chip interconnections, as they offer low temperature processing and simplistic fabrication steps as compared to conventional metallic solders, and are potentially less harmful to the environment. Unlike metallic solders, conductive polymers hold their shape when heated, thus making it possible to define bumps with a finer pitch.
Using photolithography to form flip chip bumps would not only improve the placement accuracy (bumping resolution), but would also allow for control of the height of the bump in order to cater to various applications, e.g., high-speed, high-power, etc. Application of this idea to replace screen-printing techniques has not received much success in the past, since this requires the development of sophisticated head equipment for dispensing and scraping of the polymer material. Such dispensing and scraping is necessary since baking the paste and photoresist together at high temperatures makes it difficult to selectively strip off the photoresist.
What is needed then is an alternative to the screen-printing technique used in the micro/optoelectronic packaging industry. What is further needed is an improved bump resolution as compared to screen-printing. What is still further needed is the obviation of the need for sophisticated head equipment that would otherwise be required to repeatably and accurately dispense and scrape the polymer bump material. What is even further needed is a method for commercial mass-fabrication of flip-chip interconnections which supports the use of relatively large wafers, e.g., up to 16 inches in diameter, by allowing a polishing process to remove excess bump-forming material uniformly over large wafers.